1. Field of the Invention
The present invention relates to a method for producing a semiconductor light emitting device, and specifically a method for producing a semiconductor light emitting device including a nitride semiconductor layer as a light emitting layer on a silicon substrate; and a semiconductor light emitting device produced by such a method.
2. Description of the Related Art
A light emitting device using a nitride semiconductor material, such as GaN, InN, AlN, or a mixed crystal thereof, usually includes a nitride semiconductor layer formed of, for example, InxGa1-xN crystals, as a light emitting layer on a sapphire substrate.
Recently, silicon (Si) substrates which are less expensive and have a larger area than a sapphire substrate have been produced. A nitride semiconductor light emitting device can be produced at lower cost by using such an Si substrate instead of a sapphire substrate.
A nitride semiconductor light emitting device produced using an Si substrate has the following problem. A nitride semiconductor layer has a larger thermal expansion coefficient than that of an Si substrate. When the temperature is once raised for epitaxial growth and then is lowered to room temperature, the nitride semiconductor layer shrinks more significantly than the Si substrate, due to the difference in the thermal expansion coefficient between the Si substrate and the nitride semiconductor layer.
FIG. 13 is a schematic perspective view of a nitride semiconductor light emitting device 500 using an Si substrate 91. As shown in FIG. 13, when the temperature is raised to form a nitride semiconductor layer 92 on the Si substrate 91 by epitaxial growth and then lowered to room temperature, the nitride semiconductor layer 92 significantly shrinks. As a result, tensile stress is applied to an interface between the Si substrate 91 and the nitride semiconductor layer 92, thus possibly causing cracks 93.
In the case of a nitride semiconductor light emitting device having a double-hetero structure, when the cracks 93 are generated, an invalid leak current which does not contribute to light emission is increased in magnitude. This prevents output of high luminance emission. In order to produce a nitride semiconductor device having a long life and high luminance emission, it is indispensable to prevent the generation of such cracks 93.
FIG. 14 is a schematic cross-sectional view illustrating a production step of another conventional nitride semiconductor light emitting device 600.
The nitride semiconductor light emitting device 600 is produced as follows. A mask layer 41B having a plurality of openings (windows) 42B is formed on an Si substrate 91A using an oxide layer or the like, and then a nitride semiconductor layer 92A is formed in each of the openings 42B of the mask layer 41B by epitaxial growth. Owing to such a step, a tensile stress applied to an interface between the Si substrate 91A and the nitride semiconductor layer 92A is alleviated, thus preventing the generation of cracks.
This conventional method has the following problem. Depending on the size of the mask layer 41B, the width and material of the mask layer 41B, and the growth temperature and rate, the material used for the epitaxial growth remains on the mask layer 41B. This raises the concentration of the material in a peripheral portion of the nitride semiconductor layer 92A in the opening 42B, which is in the vicinity of the mask layer 41B, is excessively high. As a result, as shown in FIG. 14, the peripheral portion of the nitride semiconductor layer 92A in the opening 42B is about three times as thick as a central portion thereof, due to growth referred to as xe2x80x9cedge growthxe2x80x9d.
As described above, the method of forming the nitride semiconductor layer 92A by epitaxial growth in the opening 42B prevents the central portion thereof from being cracked, but has a risk of causing cracks in the peripheral portion of the nitride semiconductor layer 92A due to the local distortion applied to the thick portion.
When a substrate formed of a material having a smaller thermal expansion coefficient than a nitride semiconductor material, such as Si, it is difficult to produce a nitride semiconductor light emitting device having a long life and high luminance emission, with prevention of crack generation. It is not sufficient to form a nitride semiconductor layer in an opening by epitaxial growth.
According to one aspect of the invention, a method for producing a semiconductor light emitting device including at least one first column-like multi-layer structure provided on a substrate and containing nitride-based semiconductor compound semiconductor layers represented by the general formula InxGayAlzN (where x+y+z=1, 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6z xe2x89xa61) is provided. The method includes a first step of forming a plurality of grooves in the substrate; and a second step of forming a plurality of first column-like multi-layer structures on the substrate so as to be separated by the grooves.
In one embodiment of the invention, the method further includes a third step of, after the second step, removing a substance including epitaxial layers deposited in the grooves; and a fourth step of, after the third step, forming an insulating layer in the grooves for electrically separating the plurality of first column-like multi-layer structures from each other.
In one embodiment of the invention, the method further includes the step of, after the fourth step, forming a transparent electrode for electrically connecting the plurality of first column-like multi-layer structures to each other.
In one embodiment of the invention, the method further includes the steps of, after the fourth step, forming a transparent electrode for each of the plurality of first connecting the column-like multi-layer structures; and dividing the resultant laminate into a plurality of chips such that each chip includes one first column-like multi-layer structure.
In one embodiment of the invention, the grooves are arranged in a lattice pattern. The substrate includes a plurality of first areas and a plurality of second areas, each of which is surrounded by the grooves. The plurality of first areas and the plurality of second areas are arranged in a chess board pattern. The method further comprises the steps of, before the first step, forming a mask layer so as to cover the substrate, and removing a portion of the mask layer corresponding to the grooves which are to be formed in the substrate. The second step includes the steps of removing portions of the mask layer which are on the plurality of first areas and forming one first column-like multi-layer structure on each of the plurality of first areas, and removing portions of the mask layer which are on the plurality of second areas and forming a second column-like multi-layer structure on each of the plurality of second areas.
In one embodiment of the invention, the plurality of first column-like multi-layer structures each have a thermal expansion coefficient which is larger than a thermal expansion coefficient of the substrate.
In one embodiment of the invention, the substrate is formed of silicon.
In one embodiment of the invention, the grooves each have a depth which is at least 50% of a thickness of each of the plurality of first column-like multi-layer structures in a direction vertical to a surface of the substrate, and is 10 xcexcm or less. The grooves each have a width which is 2 xcexcm or more and 10 xcexcm or less.
In one embodiment of the invention, grooves cross each other.
According to another aspect of the invention, a semiconductor light emitting device produced by the above-described method is provided.
Thus, the invention described herein makes possible the advantages of providing a method for producing a semiconductor light emitting device using an Si substrate and still preventing cracks from being generated at an interface between the Si substrate and a nitride semiconductor layer; and a semiconductor light emitting device produced by such a method.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.